Photoinduced Solid Phase Transformation in Vanadium Dioxide Films
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Photoinduced Solid Phase Transformation in Vanadium Dioxide Films S. Lysenko, V. Vikhnin, A. Rua, F. Fernandez and H. Liu Department of Physics, University of Puerto Rico, Mayaguez, PR 00681, USA
ABSTRACT Laser induced insulator-to-metal phase transition in VO2 thin films was explored using femtosecond optical pump-probe spectroscopy. The transient reflection and relaxation processes in VO2 were observed during phase transition on 10-11-10-9 temporal-scale. Optical generation of carriers in VO2 initiates extremely fast changes in reflectivity which is strongly dependent on pump fluence. Analysis of transient optical properties allows suggesting the excitonic-controlled mechanism responsible for the light-induced phase transition in VO2. INTRODUCTION The VO2 thin films exhibit thermochromic effect of insulator-to-metal (I-M) phase transition (PT) with a characteristic hysteresis at the temperature near 340 K. The ferroelastic deformation of the lattice symmetry from a monoclinic to a tetragonal (rutile) structure can be employed in ultrafast electronic and optical switching, memory, bistable electrooptical devices, optically controlled laser cavity infrared mirrors [1-4]. The extremely fast I-M PT on a 100 fs time scale can be realized by light without thermal heating of the lattice [5]. The different PT mechanisms are considered for the cases of photo- and thermally-induced PTs. Recent ultrafast optical study of PT in VO2 [5] has demonstrated bandlike character of PT. The thermal mechanism of PT implies the spontaneous charge transfer from oxygen to vanadium ion which is the driving force of PT from metallic to insulator phase [3]. The photo-induced phase transformation can be considered as PT caused by electron-electron and electron-lattice interactions and exciton-controlled mechanism [5]. In this paper we show the dynamics of photo-induced phase transition on a pico- and nanosecond time scales. The excitonic-controlled light-induced I-M PT mechanism is proposed as main origin of PT. The atomic force microscopy was applied to obtain structural parameters of VO2 films prepared by pulsed laser deposition on the amorphous (SiO2) and single crystal (MgO) substrates. An essential role of substrate to the film morphology and excited state dynamics are discussed.
EXPERIMENTAL Materials The VO2 films was fabricated by PLD technique by using Lambda Physic 110 excimer KrF laser (wavelength λ=248 nm, pulse duration of ~10-ns, 10 Hz repetition rate, 14.1 J/cm2 fluence). The 200 nm and 100 nm thick films have been formed on amorphous SiO2 and crystalline MgO substrates, respectively, from the straight flux of torch particles during 60 min in the atmosphere of O2 and Ar, under 30 mTorr pressure in the chamber. The substrate
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temperature was electrically fixed at 820 K. Oxygen and argon gases were injected into chamber through the separated jets with 10:15 and 20:5 (O2:Ar, cm3/min:cm3/min) flow speed rates for VO2/MgO and VO2/SiO2 samples, respectively. The X-ray diffraction spectra of VO2 film
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